This paper estimates the reference Evapotranspiration (ET0) and Water requirements of barley (Hordeum vulgare L.) in Ksar-Chellala region, Algeria, for one dry year by using CROPWAT software. Determination of Evapotranspiration (ET) is important in application such as irrigation design, irrigation scheduling, water resource management, hydrology and cropping systems modeling. Estimation of crop water requirements of barley (CWRb) respected the methodology adopted by the service of development and management service of FAO, based on the use of software CROPWAT 8.0. The total water requirements for barley depend on a variety of target yields and crops management. The period of climatic data used is 23 years (1990-2012), the average rain in this period is 254 mm. The total rain of the dry year is 190 mm. The results of this study show, during the vegetative cycle of barley which is 6 months, the calculation of ET0 is 453 mm, the potential water which was used by the crop barley is estimated at 281.4 mm, the efficiency of rainfall is 69 mm and a total water requirements of barley (CWRb) equals to 211 mm, this amount distributed on three months coincided with important stages of development in barley. The supplementary irrigation in these conditions with optimal contents equals water requirements estimated by CROPWAT software that increases significantly grain yield of barely. Consequently, the gross irrigation water requirements (GIWR) of 1250000 ha which project to grow barley in the Algerian steppes regions are estimated at 3.77 billion and this for a dry year and a irrigation efficiency of 70%.